1. Field of the Invention
The invention relates to a mechanical servo unit of the type comprising a primary element which has a wedge portion and which acts on pressure rollers, each of which is supported on two counter surfaces, these surfaces together forming an aperture which narrows, and where one counter surface forms part of a secondary element which passes on the amplified action and where the wedge, the pressure rollers and the counter surfaces are arranged in a cylindrical housing.
2. Description of the Prior Art
A mechanical servo unit of the type defined above is described in a British Patent No. 614905. The servo comprises a pair of pressure rollers, each pressure roller being pressed by the wedge-shaped primary element into an aperture which is formed by two further pressure rollers, the convex surfaces of these further pressure rollers forming a narrowing aperture. In one particular embodiment of this servo unit provision is also made for the wedge of the primary element to move two pairs of pressure rollers which are pressed into the aperture.
An advantage with this known servo unit is that both the pressure rollers and also the counter surfaces can be formed by standard commercial roller bodies so that these parts can easily be replaced when worn. A disadvantage however is that the transmission ratio which arises in the servo unit is not constant because of the special shape of the aperture; it is instead very progressive which means there is the risk of self-locking occuring and of the pressure rollers not following the primary element when released.
The arrangement of three or four pairs of rollers has the further disadvantage that the overall length increases in the direction of operating which is undesirable. Servo units of the type in question are typically used in machine vices and with other similar clamping devices, for example for machine tools where it is important to have the smallest possible external dimensions of the individual component parts.
Another servo unit is described in German Patent No. 2844265, which corresponds to U.S. Pat. No. 4,270,398. In the servo unit described therein, the wedge-like primary element acts on two pairs of rollers which are arranged in a narrowing aperture. The advantage of this type of design is that the transmission ratio does not change because the counter surfaces of the roller bodies are even and the geometrical ratios are similar in every clamping position. The two pairs of rollers which roll on one another and, to a small extent, also slide on each other, do however require a relatively long wedge. For this reason and due to the arrangement of two pairs of rollers a not inconsiderable overall length arises which is undesirable.
In German Patent No. 8003004, which corresponds to U.S. Pat. No. 4,384,707, a mechanical servo unit is described in which a fairly large number of balls are arranged between two discs with conical faces, said balls being pressed by a primary element with a conical pressure face into the gap between the conical counter surfaces. The arrangement here is done such that the balls are not only pressed radially outwards, but are at the same time also rotated. However, the construction with rotating, ball-shaped pressure bodies leads to considerable wear in the counter surfaces which is particularly due to the fact that the balls have a punctiform bearing surface.
A similar arrangement with which balls are also provided as pressure bodies is described in German Patent Specification 2741166, which corresponds to U.S. Pat. No. 4,347,753. An attempt is made to reduce the specific loading of the surfaces which act together with the balls by using a fairly large number of balls, which are arranged on a comparatively large diameter. However, this arrangement is only suitable for relatively short power strokes.